Submerged arc welding (also called SAW or sub arc) is a type of arc welding where the arc is not visible. Sub arc welding produces coalescence of metals by heating them with an arc between a bare metal electrode and the work piece. The arc and molten metal are submerged in a blanket of granular fusible flux on the work piece. Filler metal is provided by the electrode (or from a supplemental source such as a welding rod or metal granules). The arc is covered by the flux.
Tubular wire welding is a process in which the filler metal or flux is provided within the hollow portion of a tubular wire. Generally, tubular wire welding includes flux cored wire welding and metal cored wire welding. Both sub arc and tubular wire arc welding often are performed at relatively high output currents, such as up to 500 amps or more. Sub arc and tubular wire are welding both generally have a wire fed into an arc with an additional flux or filler metal provided. As used herein, sub arc/tubular wire welding refers to sub arc welding or tubular wire welding.
Many sub arc and tubular wire applications are automatic welding applications where either the work piece is moved under the weld head or the weld head is moved over the stationary work piece. Such automatic systems include wire feeders and are well known in the art. Wire feeders used in sub arc welding may be either constant speed or variable speed. Constant speed wire feeders are typically used with CV power supplies, and variable speed wire feeders may be used with CC power supplies. Each type of wire feeder has advantages and disadvantages. Preferably, a welding power supply should be useable with a constant speed wire feeder, or useable with either type of wire feeder.
Early automatic sub arc welding applications provided a DC output and used power sources with drooping V-A characteristics and voltage following wire electrode feeders. Subsequently, constant voltage (CV) DC sources were introduced to the process and linked to constant speed wire electrode feeders. However, magnetic fields generated by the DC arc current and surrounding the arc and the field associated with the ground currents react with each other in an unpredictable manner, causing the arc to move as if the arc were being "blown" to one side. This is referred to as arc blow. This effect is most objectionable in deep grove welds where erratic movement of the arc disturbs proper formation and placement of the weld puddle. Arc blow becomes a more severe problem as the amperage increases, because magnetic fields correspondingly increase.
Arc blow is less of a problem when using an AC power supply (because there is not a DC arc current). However, a sinusoidal output does not always perform well in sub arc welding processes because the sinusoidal wave exhibits a slow zero crossover which may result in arc rectification.
Square wave welding power sources attempt to use the advantages of sinusoidal AC welding, but with a rapid zero crossing to avoid arc rectification. One known square wave welding power supply is described in U.S. Pat. No. 4,038,515 issued to Risberg. This power supply provides for a square wave AC welding output. The Risberg design provides a constant current (CC) output and thus cannot be used with a constant speed wire feeder. The output of this power supply is at a frequency equal to the input frequency.
Another prior art sub arc welding power supply is described in U.S. Pat. No. 4,322,602 which was issued to Grist, and was owned by the assignee of the present invention. Grist describes an AC constant potential (CV) power source which may be used for sub arc welding. The output of Grist is an AC/CV output having a frequency equal to the input frequency, and having a fast zero crossing. This power supply is used with a constant speed wire feeder.
A TIG (Tungsten inert gas) welding power supply is described in U.S. Pat. No. 5,340,963, which is also owned by the assignee of the present invention, and is hereby incorporated by reference. U.S. Pat. No. 5,340,963 shows an AC power source for welding which receives a three phase input and provides a single phase AC output, having relatively fast zero crossings, at a frequency 1.5 times the input frequency. This is a type of step-up cycloconverter. However, this prior art does not teach a CV mode of operation, nor a CC controller. This prior art can be operated in a DC mode, but only operates on half of the sinusoidal input (thus, the SCRs and secondary windings must be able to handle twice the current, relative to the current capacity needed if the entire input were used). This can be costly and add weight and size to the machine.
A "step-up cycloconverter", as used herein, is a cycloconverter having an output frequency greater than the input frequency. It receives an AC input at a given frequency and provides an AC output at a higher frequency. This conversion is obtained by phase control or without using switches that are forced off, such as force commutated SCRS, IGBTs or FETs. Thus, a rectifier followed by an inverter or buck/boost converter is not a cycloconverter. The applicants have learned that sub arc welding performed at a frequency greater or less than the input line frequency (50 or 60 Hz) will provide a better weld. Power sources that provide an output at greater that than 60 Hz are known and are generally inverters or other converters. However inverter based converters require the use of expensive switches that may be turned off, such as IGBT's. This is particularly true in applications such as sub arc welding where the current desired may exceed 1000 amps. Accordingly, inverter based power supplies for use in sub arc welding may be expensive and not practical.
Additionally, it is desirable to provide flexibility in a welding power supply so that it may be used for a variety of applications. For example, it is desirable to provide a welding power supply that provides an AC or DC output. Also, it is desirable to provide a welding power supply that provides either a CV or a constant current (CC) output, that may be used with a constant or variable speed wire feeder. Further, a CV type power supply is easier for the user to set up (select operating conditions and parameters). Inverter based welding power supplies may be AC/DC and CC/CV, but as described above, they may be expensive, and not appropriate for sub arc applications.
Accordingly, it is desirable to provide a welding power supply that is suitable for sub arc welding that maybe operated either a CC, or a CV mode. Also, such a power supply will preferably be operable to provide an output having a frequency different than the input line frequency, when in the AC mode, but not require the use of IGBTs or other switches that must be turned off.
One type of sub arc/tubular wire welding involves the use of two arcs, and two wires, wherein the second arc and second wire follow closely behind the first arc and first wire (less than one inch, or close enough for the magnetic interaction to be meaningful, e.g.). This type of welding is typically done for high deposition or high speed applications.
It is generally desirable to be able to control the phase relationship (or provide phase staggering) between the power signals provided to the two arcs, because the phase relationship affects the magnetic interaction between nearby arcs. Prior art three phase power supplies could provide in-phase relationships, and 120 degree out of phase relationships using a scott-T connection. However, this provides for selection between only two different phase relationships, and requires changing the type of input connection. (As used herein, the number of phase relationships includes only phase shifts from 0 to 180 degrees, and does not include reversing the order. For example, if wire 1 is 120 degrees ahead of wire 2, that is not considered a different phase relationship from wire 2 being 120 degrees ahead of wire 1). The prior art phase staggering is not necessarily helpful because, depending upon the application and particular welding conditions, it may be desirable to have the phase relationship be close to in phase, close to out of phase, or somewhere therebetween. (As used herein, two power supplies have a variable or selectable phase relationship when the phase between the two outputs may be selected to be one out at least three discrete phase relationships between zero and 180 degrees, or any phase within a range of phases.) Thus, it is desirable for a power supply to be able to control the phase relationship so that the phase may be adapted for the particular application and welding conditions. Preferably, such phase control should be provided by a power supply that does not need to be an invertor type power supply, and more preferably, by a phase control power supply to avoid unnecessary cost and power limitations.
It is also generally desirable to be able to control the frequency at which sub arc/tubular wire arc welding is performed, because frequency affects penetration and/or other weld characteristics. The optimal frequency for a particular welding application and welding conditions may vary. Specifically, the particular desired frequency is often either greater or less than the 50 to 60 Hz typically provided by utility power supplies. Generally, invertor-based power supplies may be operated at a wide variety of frequencies. However, invertor based power supplies are expensive and difficult to design to operate at the high current magnitudes typically needed for sub arc/tubular wire welding. Accordingly, a sub arc/tubular wire welding power supply that has a variable frequency, but is not invertor based, is desirable. (As used herein, a power supply is a variable frequency power supply, or has a controllable frequency, when it is capable of providing an output having a frequency selectable from a plurality of discrete frequencies, or from any frequency within a range of frequencies). Preferably, such a welding power supply will be an scr based and/or phase controlled power supply.
Another parameter that is desirable to control for sub arc/tubular wire welding is the balance, because balance affects deposition rate, penetration and other weld characteristics. Balance, as used herein, refers to the percentage of time and/or magnitude of the positive portion of the waveform, relative to the negative portion of waveform. Some welding processes are better performed with a balance other than 50 percent. Invertor based power supplies provide balance control, however due to their expense and unsuitability for high current magnitude welding, they are not particularly useful for sub arc/tubular wire welding. Accordingly, a welding power supply that provides balance control using an SCR based power circuit, and/or phase control, is desired.
Thus, a sub arc/tubular wire arc welding power supply that provides one or more of a selectable frequency output, a selectable balance output, a CV ac output, and that is capable of being used with a second power supply having outputs that are phase staggered is desired.